1. Field of the Invention
This invention relates generally to mobile telephone systems and, more particularly, to a method of assigning a high data rate to a mobile station of a CDMA system.
2. Description of the Related Art
FIG. 1 illustrates a typical cellular based wireless communication system. Communications from base station 10 to mobile stations 12, 14 and l6 are transmitted using downlink channels 18, 20 and 22, respectively. Similarly, communications from mobile stations 12, 14 and l6 to base station 10 are transmitted using uplink channels 24, 26 and 28, respectively.
In a CDMA packet data system, the base station assigns channels for high data rate transmissions by the mobile stations on the reverse link. Determination of the acceptable rate is based on a number of factors such as availability of hardware resources, sufficient back haul capacity, scheduling algorithm, the ability of the air interface to support high rate transmission and the like.
A CDMA carrier is shared by multiple mobile stations involved in voice or data communication. Each mobile station transmits a spread spectrum signal on the reverse link which interferes with the signals transmitted by all other mobile stations. Data channels can have transmission rates that are significantly higher than voice or low speed data channels. As a result, transmission power and the consequent interference produced by high rate channels is higher than that produced by voice and low speed data channels. The base station tries to minimize the impact of the mutual interference between signals received from different mobile stations through the power control procedure. To keep performance of existing voice and data users at their target quality of service levels while preserving the design coverage, a need exits to complement the existing power control method with a method which can limit the adverse impact of high data rate transmissions.